Wheels slip on a road surface when a degree of braking is increased above a certain level during the braking of the vehicle such as an automobile. When the slip is increased, a friction coefficient acting between the wheel and the road surface turns downward after a maximum value. Therefore, when the braking is suddenly performed, a braking distance between where the vehicle stops and where the braking is started, becomes longer than a minimum braking distance that can be obtained by optimum braking. When the vehicle is braked, a vertical load of the wheel moves from a rear wheel side to a front wheel side to decrease the vertical load of the rear wheel, and a braking power of the rear wheel is decreased, while the vertical load of the front wheel is increased. Although the braking power that can be obtained by the front wheel is increased, there is a limitation to the increase in braking power caused by the increase in vertical load. Accordingly, when the degree of braking is increased above a certain level, braking ability is degraded as a whole in the front and rear wheels.
In a brake control apparatus including an Anti-lock braking systems (“ABS”), Patent Document 1 proposes a braking apparatus in order to prevent rear lift-up to improve vehicle stability. In the braking apparatus, a determination of high-G braking is made when a vehicle body deceleration detecting means detects that vehicle body deceleration exceeds a predetermined value after braking starts, a determination of sudden braking is made when an amount of temporal change of the vehicle body deceleration exceeds a predetermined value, and the braking power is moderately increased to the front wheel when the determinations of the high-G braking and sudden braking are made. Patent Document 2 discloses a braking apparatus in order that vehicle running is stabilized by starting a braking power distribution control in proper timing during the sudden braking. In the braking apparatus, a vehicle speed is detected from a wheel speed, the vehicle body deceleration is computed by differentiating the vehicle speed, and a vehicle body deceleration differential value is computed by further differentiating the vehicle body deceleration. When the vehicle body deceleration differential value reaches a predetermined threshold value, a determination that a stepping-on speed of a brake pedal is too fast is made to start the braking power distribution control in order to suppress the braking power of the rear wheel. Patent Document 3 discloses a braking apparatus in order that a good attitude of the vehicle is maintained by suppressing the lift-up of the vehicle body or nose-dive during the braking. In the braking apparatus, an ideal braking power distribution line indicating a braking power distribution relationship of front and rear wheels is assumed based on an ideal braking power distribution characteristic, a braking power distribution on a real braking power distribution line and a braking power distribution on the ideal braking power distribution line are compared when the deceleration is arbitrarily generated, a ratio of the braking power distribution of the front wheel in the former is set lower than a ratio of the braking power distribution of the front wheel in the latter, and the braking power distribution among the front and rear wheels is controlled based on the ratios.    Patent Document 1: Japanese Patent Application Laid-open No. 6-255468    Patent Document 2: Japanese Patent Application Laid-open No. 2007-15494    Patent Document 3: Japanese Patent Application Laid-open No. 2007-216771